Thyristors may be used to control current flow between a source of AC electrical energy and a load. When the load is inductive, as is the case for a motor, the current flow to the motor will lag the voltage applied to the motor. Timing of the thyristors may be accomplished relative to the event of zero crossing of the voltage applied to the motor, or may be accomplished relative to the event of current flow to the motor reaching zero.
A continuing problem in the design of thyristor motor controllers is to provide an optimum means for timing the firing of the thyristors. Particularly, in some applications it is desirable to have a predetermined OFF TIME during the time interval between current reaching zero and initiation of conduction of the thyristors for the next half cycle. Such a desired OFF TIME may be used to implement control of the power factor of the motor by reducing the voltage applied to the motor. The voltage applied to the motor may be reduced in order to adjust the current lag, and thus the power factor, for efficient operation of a lightly loaded motor. When a desired OFF TIME is employed and firing of the thyristors is measured from zero crossing of the voltage applied to the motor, instabilities in motor speed may develop. However, if firing of the thyristors is measured from the time that current flow to the motor reaches zero, then starting the motor becomes difficult because no current is flowing to the motor before the motor is started.